Final answer:
The thymus is the organ that is more active in children to enhance immune function and decreases in size after puberty. It is located behind the sternum, and is responsible for the maturation and development of T cells through hormones called thymosins.
Step-by-step explanation:
The organ that is more active in children to enhance the immune function and decreases in size after puberty is the thymus. The thymus is an essential part of the immune system and is responsible for the maturation of T cells, which are crucial for the body's adaptive immunity. This organ is located behind the sternum and is most prominent during infancy, gradually shrinking in size as a person reaches adulthood. This process, known as thymic involution, indicates a shift in the immune system's focus from developing new T cells to relying on the T cells and immunological memory that were developed earlier in life.
The hormones produced by the thymus, referred to as thymosins, play a significant role in the differentiation and development of T cells. Although the thymus diminishes in size and function over time, it is essential during early childhood as it helps build the foundation for a strong immune system capable of fighting infections and diseases.